1. Field of the Invention
This invention relates to semiconductor devices and, more particularly, to photovoltaic devices.
2. Art Background
Intensive research has centered on the conversion of solar radiation into electricity. One means being explored for this conversion is the use of photovoltaic devices having an electrolyte/semiconductor interface. The sophistication of these electrolyte/semiconductor devices has rapidly increased. For example, early devices generally, although quite promising, exhibited severe interactions between the electrolyte and the semiconductor material, causing the semiconductor material to substantially degrade. The degradation that occurs is typified by the results observed when a Fe(CN).sub.6.sup.-4 /Fe(CN).sub.6.sup.-3 electrolyte is used in conjunction with a CdS semiconductor material. (See H. Gerischer, Journal of Electroanalytical Chemistry, 58, 263 (1975).)
A variety of approaches has been employed to decrease the corrosion associated with electrolyte/semiconductor interfaces. For example, if a redox couple is used that competes efficiently with the decomposition or corrosion reaction, then the electrode becomes more stable. This phenomenon is employed when CdS is used in conjunction with a polysulfide electrolyte. (See A. B. Ellis et al., Journal of the American Chemical Society, 98, 1685 and 6855 (1976).) In such case the corrosion, as compared to that occurring in the Fe(CN).sub.6.sup.-4 /Fe(CN).sub.6.sup.-3 electrolyte, is substantially reduced through the competing reaction 2e.sup.- +S.sub.n.sup.2- .revreaction.S.sub.n-1.sup.2- +S.sup.2-.
Recently, the use of a vanadium containing electrolyte together with an InP cathode has also shown substantially improved corrosion resistance. (See A. Heller et al., Journal of the American Chemical Society, 102, 6555 (1980).) However, although these improvements have significantly increased the reliability of electrolyte/semiconductor photovoltaic devices, even more stable devices would still be advantageous.
Not only has the corrosion resistance of electrolyte/semiconductor cells significantly increased, but also the solar conversion efficiencies of these cells has followed a similar trend. In particular, the previously described vanadium electrolyte in conjunction with a single crystalline InP electrode yields efficiencies of approximately 9.4% at 110 mW/cm.sup.2 illumination (approximately air mass 1). Efficiencies of this magnitude are a significant improvement from early cells. Obviously, however, even more improved efficiencies are always desirable.